Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Olefin gas cracker systems are normally designed to crack ethane, propane and on occasion butane, but typically lack the flexibility to crack heavier liquid feedstocks, particularly those that produce tar in amounts greater than one percent. As gas feeds tend to produce little tar, primary, secondary, and even tertiary transfer line exchangers (TLEs) are utilized to recover energy through the generation of high pressure and medium pressure steam, as the furnace effluent cools from the furnace outlet to the quench tower inlet. The process gas is normally then fed to a quench tower wherein the process gas is further cooled by direct contacting with quench water.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipe still bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 595° C. (1100° F.). The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which leads to rapid coking in the radiant section of the furnace, often requiring costly shutdowns for cleaning.
Steam crackers designed to operate on gaseous feedstocks, while limited in feedstock flexibility, require significantly lower investment when compared to liquid feed crackers designed for naphtha and/or heavy feedstocks that produce higher amounts of tar and byproducts. However, as may be appreciated, when the price of natural gas is high relative to crude, gas cracking tends to be disadvantaged when compared with the cracking of virgin crudes and/or condensates, or the distilled liquid products from those feeds. (e.g., naphtha, kerosene, field natural gasoline, etc). In such an economic environment, it would be desirable to extend the range of useful feedstocks to include liquid feedstocks that yield higher levels of tar.
Advantaged steam cracking feeds frequently contain asphaltenes, which lay down as coke in the convection section of conventional pyrolysis furnaces. Contaminated condensates and full range virgin gas oils (FRVGO) with up to 400 ppm asphaltenes are typical of such advantaged feeds. However, feeds with greater than 100 ppm asphaltenes cause the thickness of the coke layer to increase rapidly in part because the coke produced by the asphaltenes typically is found within a few rows of the heat exchange tubes of the convection section. Since pressure drop is a strong function of tubing diameter, a fast growing coke layer causes the convection section pressure drop to increase rapidly. For example, a one-half inch layer of coke in a five inch diameter tube triples the pressure drop across the tube, while the same one-half inch layer of coke in a three inch diameter tube increases the pressure drop by nine times. As such, it would be desirable to have an improved process for decoking a furnace for cracking a hydrocarbon feed to facilitate the use of advantaged steam cracking feeds.
G.B. Patent No. 1,306,962 proposes a process for thermally cracking hydrocarbons wherein on-stream decoking is employed. It is asserted that an advantage of more frequent decoking is that decoking will be accomplished more readily since the coke will not have had a chance to calcine over a long period. It is believed that the process proposed relates to decoking a radiant section of a furnace, rather than dealing with the unique issues of convection section decoking.
U.S. Pat. No. 5,536,390 proposes the thermal decoking of cracking gas coolers that operate with low gas pressure. This is said to be accomplished by controlling the temperature of a cleaning gas delivered to the cooler. The temperature control is said to be achieved by mixing a cleaning gas, which has been heated in a cracking oven, with a stream of relatively cool cleaning gas upstream of the cooler.
U.S. Patent Publication No. 2006/0249428 proposes a process for steam cracking heavy hydrocarbon feedstocks containing non-volatile hydrocarbons. The process includes the steps of heating the heavy hydrocarbon feedstock, mixing the heavy hydrocarbon feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase, and varying the amount of the fluid and/or the primary dilution steam stream mixed with the heavy hydrocarbon feedstock in accordance with at least one selected operating parameter of the process, such as the temperature of the flash stream before entering the flash drum.
Despite these advances in the art, there is a need for an improved process for decoking a furnace for cracking a hydrocarbon feed.